Cool Gas in High-Redshift Galaxies Carilli, C.L; Walter, F
Annual review of astronomy and astrophysics,
08/2013, Letnik:
51, Številka:
1
Journal Article
Recenzirano
Over the past decade, observations of the cool interstellar medium (ISM) in distant galaxies via molecular and atomic fine structure line (FSL) emission have gone from a curious look into a few ...extreme, rare objects to a mainstream tool for studying galaxy formation out to the highest redshifts. Molecular gas has been observed in close to 200 galaxies at
z
> 1, including numerous AGN host-galaxies out to
z
∼ 7, highly star-forming submillimeter galaxies, and increasing samples of main-sequence color-selected star-forming galaxies at
z
∼ 1.5 to 2.5. Studies have moved well beyond simple detections to dynamical imaging at kiloparsec-scale resolution and multiline, multispecies studies that determine the physical conditions in the ISM in early galaxies. Observations of the cool gas are the required complement to studies of the stellar density and star-formation history of the Universe as they reveal the phase of the ISM that immediately precedes star formation in galaxies. Current observations suggest that the order of magnitude increase in the cosmic star-formation rate density from
z
∼ 0 to 2 is commensurate with a similar increase in the gas-to-stellar mass ratio in star-forming disk galaxies. Progress has been made in determining the CO luminosity to H
2
mass conversion factor at high
z
, and the dichotomy between high versus low values for main-sequence versus starburst galaxies, respectively, appears to persist with increasing redshift, with a likely dependence on metallicity and other local physical conditions. There may also be two sequences in the relationship between star-formation rate and gas mass: one for starbursts, in which the gas consumption timescale is short (a few 10
7
years), and one for main sequence galaxies, with an order of magnitude longer gas consumption timescale. Studies of atomic FSL emission are rapidly progressing, with some tens of galaxies detected in the exceptionally bright C
ii
158-μm line to date. The C
ii
line is proving to be a unique tracer of galaxy dynamics in the early Universe and, together with other atomic FSLs, has the potential to be the most direct means of obtaining spectroscopic redshifts for the first galaxies during cosmic reionization.
Observations have set the first constraints on the epoch of reionization (EoR), corresponding to the formation epoch of the first luminous objects. Studies of Gunn-Peterson (GP) absorption indicate a ...rapid increase in the neutral fraction of the intergalactic medium (IGM) from x
HI
< 10
−4
at z ≤ 5.5, to x
HI
> 10
−3
, perhaps up to 0.1, at z∼6, while the large scale polarization of the cosmic microwave background (CMB) implies a significant ionization fraction extending to higher redshifts, z∼11 ± 3. These results, as well as observations of galaxy populations, suggest that reionization is a process that begins as early as z∼14, and ends with the “percolation” phase at z∼6 to 8. Low luminosity star-forming galaxies are likely the dominant sources of reionizing photons. Future low-frequency radio telescopes will make direct measurements of HI 21-cm emission from the neutral IGM during the EoR, and measurements of secondary CMB temperature anisotropy will provide details of the dynamics of the reionized IGM.
We present an analysis of the energetics and particle content of the lobes of 24 radio galaxies at the cores of cooling clusters. The radio lobes in these systems have created visible cavities in the ...surrounding hot, X-ray- emitting gas, which allow direct measurement of the mechanical jet power of radio sources over six decades of radio luminosity, independently of the radio properties themselves. We find that jet (cavity) power increases with radio synchrotron power approximately as image, where image depending on the bandpass of measurement and state of the source. However, the scatter about these relations caused by variations in radiative efficiency spans more than 4 orders of magnitude. A number of factors contribute to this scatter, including aging, entrainment, variations in magnetic field strengths, and the partitioning of energy between electrons and nonradiating heavy particles. After accounting for variations in synchrotron break frequency (age), the scatter is reduced by sim50%, yielding the most accurate scaling relation available between the lobe radio power and the jet (cavity) power. Furthermore, we place limits on the magnetic field strengths and particle content of the radio lobes using a variety of X-ray constraints. We find that the lobe magnetic field strengths vary between a few to several tens of microgauss depending on the age and dynamical state of the lobes. If the cavities are maintained in pressure balance with their surroundings and are supported by internal fields and particles in equipartition, the ratio of energy in electrons to heavy particles (k) must vary widely from approximately unity to 4000, consistent with heavy (hadronic) jets.
We present evidence for very high gas fractions and extended molecular gas reservoirs in normal, near-infrared-selected (BzK) galaxies at z ∼ 1.5. Our results are based on multi-configuration CO2–1 ...observations obtained at the IRAM Plateau de Bure Interferometer. All six star-forming galaxies observed were detected at high significance. High spatial resolution observations resolve the CO emission in four of them, implying sizes of the gas reservoirs of order of 6–11 kpc and suggesting the presence of ordered rotation. The galaxies have UV morphologies consistent with clumpy, unstable disks, and UV sizes that are consistent with those measured in CO. The star formation efficiencies are homogeneously low within the sample and similar to those of local spirals—the resulting gas depletion times are ∼0.5 Gyr, much higher than what is seen in high-z submillimeter galaxies and quasars. The CO luminosities can be predicted to within 0.15 dex from the observed star formation rates (SFRs) and stellar masses, implying a tight correlation of the gas mass with these quantities. We use new dynamical models of clumpy disk galaxies to derive dynamical masses for our sample. These models are able to reproduce the peculiar spectral line shapes of the CO emission. After accounting for the stellar and dark matter masses, we derive molecular gas reservoirs with masses of (0.4–1.2)×1011 M☉. The implied conversion (CO luminosity-to-gas mass) factor is very high: αCO = 3.6 ± 0.8, consistent with a Galactic conversion factor but 4 times higher than that of local ultra-luminous IR galaxies that is typically used for high-redshift objects. The gas mass in these galaxies is comparable to or larger than the stellar mass, and the gas accounts for an impressive 50%–65% of the baryons within the galaxies' half-light radii. We are thus witnessing truly gas-dominated galaxies at z ∼ 1.5, a finding that explains the high specific SFRs observed for z > 1 galaxies. The BzK galaxies can be viewed as scaled-up versions of local disk galaxies, with low-efficiency star formation taking place inside extended, low-excitation gas disks. These galaxies are markedly different than local ULIRGs and high-z submillimeter galaxies and quasars, where higher excitation and more compact gas is found
The largest galaxies in the universe reside in galaxy clusters. Using sensitive observations of carbon monoxide, we show that the Spiderweb galaxy—a massive galaxy in a distant protocluster—is ...forming from a large reservoir of molecular gas. Most of this molecular gas lies between the protocluster galaxies and has low velocity dispersion, indicating that it is part of an enriched intergalactic medium. This may constitute the reservoir of gas that fuels the widespread star formation seen in earlier ultraviolet observations of the Spiderweb galaxy. Our results support the notion that giant galaxies in clusters formed from extended regions of recycled gas at high redshift.
The VLA-COSMOS Large Project is described and Its scientific objective is discussed. We present a catalog of similar to 3600 radio sources found in the 2 deg super(2) COSMOS field at 1.4 GHz. The ...observations in the VLA A and C configuration resulted in a resolution of 1.5" x 1.4" and a mean rms noise of similar to 10.5 (15) mu Jy beam super(-1) in the central 1 (2) deg super(2). Eighty radio sources are clearly extended consisting of multiple components, and most of them appear to be double-lobed radio galaxies. The astrometry of the catalog has been thoroughly tested, and the uncertainty in the relative and absolute astrometry are 130 and <55 mas, respectively.
The H I 21 cm transition line is expected to be an important probe into the cosmic dark ages and epoch of reionization. Foreground source removal is one of the principal challenges for the detection ...of this signal. This paper investigates the extragalactic point source contamination and how accurately bright sources (1 Jy) must be removed in order to detect 21 cm emission with upcoming radio telescopes such as the Murchison Widefield Array. We consider the residual contamination in 21 cm maps and power spectra due to position errors in the sky model for bright sources, as well as frequency-independent calibration errors. We find that a source position accuracy of 0.1 arcsec will suffice for detection of the H I power spectrum. For calibration errors, 0.05% accuracy in antenna gain amplitude is required in order to detect the cosmic signal. Both sources of subtraction error produce residuals that are localized to small angular scales, k 0.05 Mpc--1, in the two-dimensional power spectrum.
We present the VLA-COSMOS 3 GHz Large Project based on 384 h of observations with the Karl G. Jansky Very Large Array (VLA) at 3 GHz (10 cm) toward the two square degree Cosmic Evolution Survey ...(COSMOS) field. The final mosaic reaches a median rms of 2.3 μJy beam-1 over the two square degrees at an angular resolution of 0.75″. To fully account for the spectral shape and resolution variations across the broad (2 GHz) band, we image all data with a multiscale, multifrequency synthesis algorithm. We present a catalog of 10 830 radio sources down to 5σ, out of which 67 are combined from multiple components.Comparing the positions of our 3 GHz sources with those from the Very Long Baseline Array (VLBA)-COSMOS survey, we estimate that the astrometry is accurate to 0.01″ at the bright end (signal-to-noise ratio, S/N3 GHz > 20). Survival analysis on our data combined with the VLA-COSMOS 1.4 GHz Joint Project catalog yields an expected median radio spectral index of α = −0.7. We compute completeness corrections via Monte Carlo simulations to derive the corrected 3 GHz source counts. Our counts are in agreement with previously derived 3 GHz counts based on single-pointing (0.087 square degrees) VLA data. In summary, the VLA-COSMOS 3 GHz Large Project simultaneously provides the largest and deepest radio continuum survey at high (0.75″) angular resolution to date, bridging the gap between last-generation and next-generation surveys.
We present high-resolution observations of the 880 mum (rest-frame FIR) continuum emission in the z= 4.05 submillimeter galaxy GN20 from the IRAM Plateau de Bure Interferometer (PdBI). These data ...resolve the obscured star formation (SF) in this unlensed galaxy on scales of 0".3 x 0".2 (~2.1 x 1.3 kpc). The observations reveal a bright (16 + or - 1 mJy) dusty starburst centered on the cold molecular gas reservoir and showing a bar-like extension along the major axis. The striking anti-correlation with the Hubble Space Telescope/Wide Field Camera 3 imaging suggests that the copious dust surrounding the starburst heavily obscures the rest-frame UV/optical emission. A comparison with 1.2 mm PdBI continuum data reveals no evidence for variations in the dust properties across the source within the uncertainties, consistent with extended SF, and the peak star formation rate surface density (119 + or - 8 M sub(middot in circle) yr super(-1) kpc super(-2)) implies that the SF in GN20 remains sub-Eddington on scales down to 3 kpc super(2). We find that the SF efficiency (SFE) is highest in the central regions of GN20, leading to a resolved SF law with a power-law slope of Sigma sub(SFR)~ (ProQuest: Formulae and/or non-USASCII text omitted), and that GN20 lies above the sequence of normal star-forming disks, implying that the dispersion in the SF law is not due solely to morphology or choice of conversion factor. These data extend previous evidence for a fixed SFE per free-fall time to include the star-forming medium on ~kiloparsec scales in a galaxy 12 Gyr ago.
CLUSTER MAGNETIC FIELDS Carilli, C. L; Taylor, G. B
Annual review of astronomy and astrophysics,
01/2002, Letnik:
40, Številka:
1
Journal Article
Recenzirano
Magnetic fields in the intercluster medium have been measured using a
variety of techniques, including studies of synchrotron relic and halo radio
sources within clusters, studies of inverse Compton ...X-ray emission from
clusters, surveys of Faraday rotation measures of polarized radio sources both
within and behind clusters, and studies of cluster cold fronts in X-ray images.
These measurements imply that most cluster atmospheres are substantially
magnetized, with typical field strengths of order 1 μGauss with high areal
filling factors out to Mpc radii. There is likely to be considerable variation
in field strengths and topologies both within and between clusters, especially
when comparing dynamically relaxed clusters to those that have recently
undergone a merger. In some locations, such as the cores of cooling flow
clusters, the magnetic fields reach levels of 10-40 μG and may be
dynamically important. In all clusters the magnetic fields have a significant
effect on energy transport in the intracluster medium. We also review current
theories on the origin of cluster magnetic fields.
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